Abstract
Mesothelin (MSLN) may be the most "dramatic" of the tumor markers, being strongly overexpressed in nearly one-third of human malignancies. The biochemical cause is unclear. We previously ascribed this cancer-specific overexpression to an element, Canscript, residing around 50 bp 5' of the transcription start site in cancer (Hucl, T., Brody, J. R., Gallmeier, E., Iacobuzio-Donahue, C. A., Farrance, I. K., and Kern, S. E. (2007) Cancer Res. 67, 9055-9065). Herein, we found a Canscript promoter activity elevated over 100-fold in cancer cells. In addition to a highly conserved TEAD1 (TEA domain family member 1)-binding MCAT motif, nucleotide substitution revealed the consensus core sequence (WCYCCACCC) of an SP1-like motif in Canscript. The unknown transcription factor binding to the SP1-like motif may hold the key for the cancer specificity of Canscript. SP1, GLI1, and RUNX1, -2, and -3 appeared unlikely to be the direct transcription factors acting at the SP1-like motif, but KLF6 had some features of such a candidate. YAP1, a TEAD1-binding protein, appeared necessary, but not sufficient, for Canscript activity; knockdown of YAP1 by small interfering RNAs greatly reduced MSLN levels in MSLN-overexpressing cells, but overexpressing YAP1 in MSLN-negative cells did not induce MSLN expression. Cansript-like sequences were found in other genes up-regulated in pancreatic cancer; reporters driven by the sequences from FXYD3, MUC1, and TIMP1 had activities more than 2 times that of the control. This suggested that the cause of MSLN overexpression might also contribute mechanistically to the overexpression of other tumor markers.